code: def test_single_step(self):
      optimizer = Ftrl(learning_rate=0.5)
      grads = np.array([1.0, 6.0, 7.0, 2.0])
      vars = backend.Variable([1.0, 2.0, 3.0, 4.0])
      optimizer.apply_gradients(zip([grads], [vars]))
      self.assertAllClose(
          vars, [0.2218, 1.3954, 2.3651, 2.8814], rtol=1e-4, atol=1e-4
      )
  
file_path: keras/keras/optimizers/ftrl_test.py
imports: import numpy
  from keras import backend
  from keras import testing
  from keras.optimizers.ftrl import Ftrl
label_desc: Use image node_id 2 for calling the FtrlTest obj's underlying member method code with example usage: obj.test_single_step() without return types